Recently, there has arisen a great demand for a way to produce wide thin ribbons of single crystal of good quality at high speed. Such crystal ribbons are used as a semiconducting material, e.g. silicon crystal or Group IV element crystal, or III-V or II-IV compound crystal, or as an insulating material, e.g. sapphire. The demand for such material is increasing due not only to an increasing use of semiconducting devices, such as diodes, transistors, integrated circuits, etc., but also because they are used for solar batteries which are now being developed extensively for the purpose of utilizing solar energy.
For these purposes, thin ribbons of single crystal are desirable, since they will eliminate the step of slicing rod-shaped crystals and make possible a continuous production line from the preparation of a crystalline material to the processing of a device to be made thereof.
In the conventional art, the crystalline material to be used as mentioned above is crystallized in the form of a cylindrical rod and then is sliced into wafers.
Before now, many different processes have been proposed for the production of crystal ribbons. However, in such a conventional process, for example in the Dendrite process, though high speed production of crystal ribbons (up to 300 mm/min) is possible, many defects or disadvantages are found; the resultant crystal ribbon is of narrow width, and inevitably includes such gross crystal defects as twin lamellae and locally-concentrated impurities. In another conventional process, e.g. the Non-Dendrite process or the EFG process, the crystal ribbons obtained are wider and have fewer defects but the rate of crystal growth is slower than in the Dendrite process. The Web process has also been proposed but is very complicated and too delicate to effect stable crystal growth. Thus, none of the conventional processes are practical for the production of a thin crystal ribbon, particularly of single crystal on a commercial scale.
Recently U.S. Pat. Nos. 3,681,033 and 3,031,275 have proposed the horizontal growth of crystal ribbons. The U.S. Pat. No. 3,681,033 granted to Bleil discloses a method of horizontally growing a crystal ribbon from a seed crystal, which comprises maintaining a melt of a crystalline substance in a crucible with the height of the melt kept above the edge of the crucible, holding the seed crystal in contact with the surface of said melt to melt the surface of the seed slightly, and then horizontally pulling said seed while contacting the upper surface of the growing crystal ribbon with a solid heat sink thereby to produce a flat crystalline ribbon of predetermined thickness. According to this process, the solidification heat generated at the crystal growing interface between the crystal body and the melt as the crystal is growing is removed perpendicularly from the upper surface of the speed and crystal ribbon grown, and thereby a thin and wide-spread cooling surface layer is easily formed on the surface of the melt. Thus, in the cases of ice and Ge, crystal ribbons of relatively large width are obtained rather successfully. However, this process uses a solid body as a heat sink, and it results in variation of cooling effect and size of crystal ribbon due to the imperfect solid-solid contact, and, moreover, is not so practical nor effective as to realize fast crystallization of ribbons. The horizontal crystal growth technique, therefore, has never been practiced on a commercial scale, because of the lower quality of the resultant crystal ribbon compared to that of the conventional ones and the low growth rate.
Bleil also suggests in another paper "soft" cooling through radiation only or by means of a gaseous medium or a liquid medium for the purpose of realizing uniform crystal growth, but says nothing about how to increase the rate of crystal growth. It is supposed that the crystal growth velocity of the Bleil patent is about 3 mm/min at the most.
In addition, in the conventional horizontal crystal growth, not so much importance was placed on the distance between the crystal growing interface and the heating surface of a heating means which heats the crystal growing interface from the side of the melt. The distance was not less than 1/4 inches (e.g. see U.S. Pat. No. 3,681,033 to Bleil). Therefore, the conventional horizontal crystal growth process was practiced under conditions resulting in very large time delays in compensating for the variation or fluctuation of temperatures at the crystal growing interface, and it was difficult to keep the temperature within a very small tolerance. This is essentially required for adjusting the crystal ribbon to predetermined shape and size. Thus, in my experiments using this method the crystal ribbons produced vary widely in width, thickness and surface flatness. The density of dislocations found in the crystal is very high and sometimes gross defects such as twin planes are also found in the crystal. Namely, the crystallinity obtained is not satisfactory. Many other disadvantages are also experienced, e.g. extremely low yield, complicated procedures for crystal growth, frequent interruption of the process, etc.